Method for operating asynchronous motors and corresponding device

ABSTRACT

Asynchronous motors are controlled by way of two- or three-phase antiductors that comprise at least two pair of thyristors connected in antiparallel and fired at certain intervals. According to the inventive method, a fundamental wave with the desired frequency is defined for the first phase and in the other phase a respective fundamental wave is defined that has the same frequency as that in the first phase but phase-shifted by corresponding values. For every phase, firing intervals are marked that have the same polarity as the respective fundamental wave. Those intervals of the potential firing intervals are used for every phase at which there is a potential firing interval in one of the two other phases. These intervals are used as the actual firing intervals.

[0001] The invention relates to a method and an associated device forfinding triggering times of two- or three-phase power controllers foroperating asynchronous motors, the power controllers being connected toa three-phase network without connecting the star points of the statorwinding of the asynchronous motor and which comprise at least two pairsof antiparallel-connected thyristors which are triggered at specifictimes.

[0002] In various drive tasks, it is desired to operate an asynchronousmotor with a rotational speed which is lower as compared with thenominal rotational speed and, in the process, to operate differentdirections of rotation, without changing the direction of rotation ofthe feeding network. For this purpose, conventional three-phase powercontrollers, such as electronic motor switch gear or soft starters, asthey are known, can cost-effectively be provided with additionalfunctionalities. Potential applications are, for example, in thepositioning of transported goods or in electrically operated gates.

[0003] U.S. Pat. No. 4,791,341 A, U.S. Pat. No. 4,524,361 A and U.S.Pat. No. 4,461,985 A already disclose equipping a three-phase powercontroller with a total of five pairs of antiparallel thyristors. Usingthis, without changing the direction of rotation of the feeding network,a reversal of the direction of rotation of the connected asynchronousmachine can be achieved. Furthermore, it is known from U.S. Pat. No.4,481,456 A to use a total of nine pairs of antiparallel thyristors insuch a way that each external conductor of the feeding network can beconnected to each terminal of the three-phase asynchronous machine via apair of the thyristors.

[0004] With this arrangement, a matrix direct inverter, as it is known,is implemented.

[0005] In addition, EP 0 408 045 B1 and EP 0 512 372 A2 disclose amethod and associated devices for motor control in which a pulse patternis specified, with which fundamental waves can be produced at afrequency which corresponds to 1/(6n+1) of the mains frequency of thefeeding network, n being a natural number. Here, therefore, fundamentalwaves can be produced whose frequencies correspond to {fraction (1/7)},{fraction (1/13)}, {fraction (1/19)} etc., of the mains frequency.However, an option to reverse the direction of rotation is notassociated with this. Finally, DE 25 58 113 A1 already discloses theproposal of producing 1/(6n+1) of the mains frequency by means ofspecific pulses. Here, in order to reverse the direction of rotation,either two further pairs of antiparallel thyristors or mechanicalchangeover devices are needed.

[0006] Starting from the prior art, it is an object of the invention tospecify a method with which suitable triggering times for a three-phasepower controller for operating an asynchronous motor can be found. Atthe same time, without additional outlay on power-electronic components,in particular inverter valves, and additional switching elements, merelyby means of a three-phase power controller having three or two pairs ofantiparallel thyristors, an operation of an asynchronous machine withany desired fundamental waves is to be produced. In particular, thefundamental wave is to correspond to 1/k the mains frequency, k being anumber≧3. Furthermore, at the same time any desired direction ofrotation is to be possible.

[0007] According to the invention, the object is achieved by thesequence of method steps according to patent claim 1. Developments arespecified in the dependent claims. An associated device for controllingan asynchronous motor in accordance with the method of the invention isspecified in patent claim 10.

[0008] In the invention, firstly, all the potentially possibletriggering pulses are determined which result in the zero crossing ofthe concatenated voltage minus an angle φ. For the triggering of thethyristor pair in the external conductor A, potential triggering pulsesresult, for example, when the phase angle of the feeding network is atthe angle φ before the zero crossing of the concatenated voltage(U_(AB)) or of the concatenated voltage (U_(CA)). In this case, theangle φ has a magnitude preferably between 30 and 60°.

[0009] In the method according to the invention, firstly, for all threeexternal conductors, the fundamental waves are defined which correspondto the desired rotational speed and direction of rotation of the motor.In particular in the case of a three-phase power controller whichcontains three pairs of antiparallel thyristors, only the potentialtriggering pulses in pairs are used, and therefore in each case twopairs of thyristors are triggered, whose resultant current flow has apolarity corresponding to the defined fundamental waves. In an entirelycorresponding manner, in the case of a three-phase power controllerwhich contains only two pairs of antiparallel thyristors, in whichtherefore thyristors are used only in two external conductors and thethird external conductor is bridged, in addition those pairs oftriggering pulses which cause a current flow only between the externalconductors that are not bridged are ruled out. In this way, anuncontrollable current in the bridged external conductor whose polaritydoes not correspond to that of the defined fundamental wave is avoided.

[0010] In the invention, it is particularly advantageous that anydesired rotational speeds with 1/k of the nominal rotational speed canbe produced, k being a number≧3. It is also advantageous that merely byvarying the angle φ, the torque output at the frequency of the definedfundamental wave can be influenced.

[0011] The method described is implemented in particular by means ofsoftware. It can therefore be implemented simply in existing three-phasepower controllers without additional expenditure on components.

[0012] Further details and advantages of the invention emerge from thefollowing figure description of an exemplary embodiment, using thedrawing in conjunction with further subclaims. In the drawing:

[0013]FIG. 1 shows a device for controlling an asynchronous motor withapplication of the method according to the invention,

[0014]FIG. 2 shows graphs to illustrate the method in the case of amotor rotating clockwise,

[0015]FIG. 3 shows a detail from FIG. 2 to illustrate the influence ofthe triggering angle,

[0016]FIG. 4 shows a graph to illustrate the method in the case of amotor running counterclockwise,

[0017]FIG. 5 shows a device corresponding to FIG. 1 specifically for thetwo-phase control of a motor,

[0018]FIG. 6 shows a graph of the method with two pairs of antiparallelthyristors and

[0019]FIG. 7 and FIG. 8 show flow diagrams for the determination bymeans of software of the triggering times for the individual phases.

[0020] In FIG. 1 and FIG. 5 in each case an induction machine 2, forexample a three-phase asynchronous machine, is connected to the phasesof the mains via a three-phase alternating current power controller 4 asa three-phase power controller, as it is known. In FIG. 1, the mains isthe phases A, B and C of a three-phase network and, in FIG. 4, thephases A and B of a two-phase network.

[0021] Each of the phases is assigned a valve arrangement, for examplein FIG. 1 each phase A, B and C is assigned a valve arrangement V1, V2,V3 and, in FIG. 5, the phases A and B are assigned a valve arrangementV1 and V2. The valve arrangements in each case comprise twoantiparallel-connected thyristors 6. The triggering electrodes of thethyristors 6 are connected to a control device, with which thetriggering signals required to trigger the thyristors 6 are provided ina predefined chronological sequence.

[0022] Between two external conductors of the network, for examplebetween the terminals A and B of the network in FIG. 1, a voltagemeasuring device 10 is connected, at the output of which the mainsvoltage U_(AB) occurring between these two terminals A and B isavailable. Furthermore, there is a control device 8 for controlling thephase gate angle for the purpose of stopping the motor softly. A controldevice of such a type is preferably implemented by a microcontroller.

[0023] In the present case, the control device 8 is used to process asuitable program, with which the operation of the device can be carriedout exclusively by means of software. In this case, the basis is astandardized method in which any desired parts of the nominal rotationalspeed (>=3) with any desired direction of rotation of the motor can beachieved for two- and three-phase soft starters. The control device canalso be a microcontroller already provided for the motor.

[0024] In FIGS. 2 and 4, the individual signals s at a rotational speedof {fraction (1/9)} of the nominal rotational speed are illustrated. Inparticular, FIG. 2 reproduces the situation in the case of a motorrotating clockwise and, in particular, FIG. 4 reproduces the situationin the case of a motor rotating counterclockwise.

[0025] In the graphical representations, the signals have the followingmeaning:

[0026] VAB=voltage of the phase A-B from FIG. 1. The signal is used as areference for the calculated times for triggering the thyristors.

[0027] IA=current in the phase A in the event of triggering 30° beforethe zero crossing of the concatenated voltage.

[0028] Since a current must always flow in two phases, for each currentpulse in one phase there is a current with opposite polarity in anotherphase, which are designated by IB and IC. The fundamental wave of thecurrent at 1/k of the nominal rotational speed is designated by FA, FBand FC for the individual phases. In each case potential triggeringtimes PFA, PFB and PFC can be derived from the fundamental wave. Thefinal triggering times are designated by DFA, DFB and DFC.

[0029] To generate the triggering times, the following is done inindividual steps: firstly, for the phase A, a fundamental wave at thedesired frequency corresponding to 1/k of the final rotational speed ofthe motor is defined. In this case, the phase shift is unimportant. Inthe case of clockwise rotation, a fundamental wave is defined for thephase B which has the same frequency as that of the phase A but istime-delayed with respect to the latter by 120°—based on the dividedfrequency. For the phase C, the same is true as for the phase A, buthere the shift is 240°.

[0030] In the second step, for each phase the triggering times whoseassociated current has the same polarity as that of the respectivefundamental wave are marked. In the following step, from the potentialtriggering times for each phase, those are used at which there is apotential triggering time in one of the two other phases. Thesetriggering times are used as the actual triggering times to operate thethree-phase power controller.

[0031]FIG. 3 shows how the triggering angle φ determines the torque ofthe asynchronous motor. With any desired devisors of the nominalrotational speed, the triggering angle φ>0 can preferably be adjustedbetween 30° and 60°, which results in a torque that can be preset.

[0032] In order to achieve a rotational movement of the motor in theinverse direction, in accordance with the method described by using FIG.2, the position of the fundamental waves of the external conductors Band C is interchanged. The selection and determination of the triggeringtimes otherwise proceeds in a way identical to that in FIG. 2, which isreproduced by using FIG. 4.

[0033] In FIG. 5, a stator for a two-phase network having two pairs ofantiparallel thyristors 6 is constructed, the third external conductorbeing permanently bridged. When the two thyristors pairs are driven, acurrent will also flow in this external conductor. For this reason,those triggering pulses which relate only to the two external conductorsfitted with thyristors are removed.

[0034] The latter is illustrated by using FIG. 6, in which again{fraction (1/9)} of the nominal rotational speed is assumed. For thecase in which the phase A is bridged, the triggering pulses which relateonly to the external conductors B and C are removed.

[0035] Using the control device 8 from FIG. 1 and FIG. 5, the respectivethyristors are driven at suitable times in order to set a predefinedrotational speed. For this purpose, the control device 8 comprises acomputing unit, which in the following text is also designated by 8 aand can be a microcontroller MC which is normally present in the case ofan up-to-date three-phase power controller, for the purpose ofdetermining the triggering times by means of software. Here, referenceis made to the graph of FIG. 4, in which a complete period of nineindividual periods is illustrated. There are counters SC, FC and CC,which count the individual times.

[0036] Each individual mains period is subdivided into 60-degreesections. These are counted by the counter SC. The counter FC counts the60-degree sections within one fundamental period. The counter CC is usedto count the mains periods within one fundamental period.

[0037] During each 60-degree section, 2 functions are used whichcalculate the polarity of the current in the respective phase and thatof the fundamental wave.

[0038] The determination of the polarity of the current (functionsignOfCurrent) is illustrated in the following table: CC 0 1 2 3 4 5Phase A −1 −1 0 1 1 0 B 1 0 −1 −1 0 1 C 0 1 0 0 −1 −1

[0039] Using FIG. 7, the determination of the polarity of thefundamental wave is illustrated, a speed factor (Speed-factor) SF=9being used: in position 50, a counter value x=FC is assumed whichcorresponds to 60° sections within a fundamental period. 100 designatesthe phase A, 200 the phase B and 300 the phase C. In the decisionelements 101, 201 and 301, a check is made on the value of the variablex. If the value is satisfied, a corresponding value based on the outputvalue FC is specified at the positions 102, 202 and 302. In the othercase, a value increased by the speed 6×SF is output at the position 103,203 and 303. In each case a corresponding shift takes place in theindividual phases. In the positions 104, 204 and 304, the values aresuperimposed and the sum signal is passed onto the position at 305. At310, a decision is made about the speed value and the polarity of thecurrents is output by using the sign.

[0040] For the triggering of the thyristors 6, the procedure is suchthat the above procedure is called every 60 degrees, based on VAB. Theexact time of the call lies an adjustable time before the 60-degreemark. The magnitude of this time interval determines the power suppliedto the motor and therefore the torque developed by the motor. By usingCC, SC, and FC, a decision is made for each phase pair as to whethertriggering is to take place in the respective phases.

[0041] In FIG. 8, by using the decision diamonds, a decision is made onthe basis of the signs of the individual phases as to which thyristorsare to trigger. Position 400 relates to the phases A and B, position 410relates to the phases B and C, and position 420 relates to the phases Aand C. Depending on the sign determined using FIG. 7, in the positions401, 411 and 421 in each case the signal for suitable triggering of thethyristors associated with the individual phases is provided.

[0042] In the examples according to the figures, it is assumed that thetriggering lies 30° before the zero crossing of the respectivelyconcatenated voltage. In order to increase the motor torque, thetriggering can be advanced, a longer current flow time being achievedfor each triggering. The triggering can preferably be carried out 30 to60° before the zero crossing of the concatenated voltage.

1. A method for finding triggering times of two- or three-phase powercontrollers for operating asynchronous motors, the three-phase powercontrollers being connected to a three-phase network without connectingthe start points of the stator winding of the asynchronous motor and thefeeding network and comprising at least two pairs ofantiparallel-connected thyristors, which are triggered at previouslydeterminable times, having the following method steps: firstly, for thepairs of antiparallel thyristors, potential triggering pulses aredefined which lie at a specific triggering angle (φ) before therespective zero crossing of the mains voltage, furthermore, a sinusoidalfundamental wave is defined at a frequency which corresponds to thedesired rotational speed of the motor and whose frequency is lower thanthat of the mains voltage, then, two further, phase-shifted fundamentalwaves are defined at the same frequency, in each case beingphase-shifted in relation to the first fundamental wave, for each phase,the previously marked potential triggering times which would cause acurrent of opposite polarity to that of the respectively associatedfundamental wave are rejected, of the remaining potential triggeringtimes, those times are selected for triggering each phase at which thereis likewise a triggering time in one of the two other phases, thetriggering times determined in this way are used as actual triggeringtimes for the thyristors of the three-phase power controllers.
 2. Themethod as claimed in claim 1, characterized in that in the case ofclockwise rotation, a fundamental wave is defined for the second phasewhich has the same frequency as the first phase, but is time-delayed inrelation to the latter by 120°, and for the third phase a fundamentalwave is defined which corresponds to that of the first phase but istime-delayed in relation to the latter by 240°.
 3. The method as claimedin claim 1, characterized in that in the case of counterclockwiserotation, a fundamental wave is defined for the second phase which hasthe same frequency as the first phase but is time-delayed in relation tothe latter by 240° and, for the third phase, a fundamental wave isdefined which corresponds to that of the first phase but is time-delayedin relation to the latter by 120°.
 4. The method as claimed in claim 1,characterized in that the frequency of the fundamental wave is 1/k ofthe nominal rotational speed of the motor, where k≧3.
 5. The method asclaimed in claim 1, characterized in that any desired devisors of thenominal rotational speed with (k≧3) are generated with any desireddirection of rotation of the motor.
 6. The method as claimed in one ofthe preceding claims, characterized in that the torque in the case ofthe arbitrary devisors k≧3 of the nominal rotational speed is influencedby the triggering angle (φ).
 7. The method as claimed in claim 6,characterized in that the triggering angle (φ) lies between 30° and 60°.8. The method as claimed in one of the preceding claims, characterizedin that the generation of the fundamental waves and the selection of thetriggering times is determined by means of software.
 9. The method asclaimed in claim 8, characterized in that the generation of thefundamental waves and the selection of the triggering point are carriedout in the microcontroller (μC) present in the controller of theasynchronous motor.
 10. A device for finding triggering times of two- orthree-phase power controllers for operating an asynchronous motor, inparticular for starting and stopping the asynchronous motor, having athree-phase power controller for the operational control of theasynchronous motor, having at least two pairs of antiparallel-connected,respectively selectively triggerable thyristors, by using the method asclaimed in claim 1 or one of claims 2 to 9, characterized by a computingunit (8 a) for the determination and selection of triggering times ofthe thyristors (6) by means of software, which has the following means:counters (SC, FC, CC) for counting 60°-sections of fundamental wavesbelonging to the individual phases, means for determining the polarityof the current at the times listed, decision means for defining thetriggering times for the individual thyristors.
 11. The device asclaimed in claim 10, characterized in that the computing unit (8 a) isthe microcontroller (μC) present in the motor (2).
 12. The device asclaimed in claim 10, characterized in that the computing unit (8 a) ispart of a control device (8) as a three-phase power controller.
 13. Thedevice as claimed in claim 12, characterized in that the computing unit(8 a) sets up a program for determining the triggering times.
 14. Thedevice as claimed in claim 13, characterized in that by using thecontrol unit (8), the driving of the thyristors (6) is carried out atthe triggering times determined by the computing unit (8 a).